Paper - Perichondrial ossification and the fate of the perichondrium with special reference to that of the otic capsule
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Perichondrial Ossification And The Fate of the Perichondrium With Special Reference to That of the Otic Capsule
T. H. Bast Department of Anatomy, University of Wisconsin, Madison
Perichondrial ossiﬁcation for most so-called cartilage or long bones is well described in most text books of histology. Long cartilage bones are foreshadowed by a growing shaft-like model of cartilage. Such a cartilage model ﬁrst appears in the mid region of the future bone. It increases in length gradually by multiplication of its cells toward the ends of the shaft. This process of cartilage cell multiplication continues until the individual has attained full growth. The shaft like model of cartilage in the young fetus is surrounded by a layer of compact mesenchyma (shown at left of ﬁg. 3) which is known as the perichondrium. These cells in this perichondium which lie next to the cartilage may themselves become cartilage cells and thus slightly increase the diameter of the model in its early stages of development.
This augmentation in diameter is short lived as the center of the early cartilaginous model becomes quickly transformed into the diaphyseal ossiﬁcation center with its enlarged lacuna and calciﬁed matrix. This diaphyseal ossiﬁcation center occurs in the early stages of the development of the cartilaginous model. Thus, new cartilage is forming at the ends of the model while the center is ossifying.
In response to this change in the cartilage model, the adjacent mesenchymal cells of the perichondrium metamorphose into osteoblasts which multiply rapidly and deposit osteoid tissue on the surface of the changed cartilage. This osteoid tissue quickly becomes calciﬁed. These steps can be seen by reading ﬁgure 3 from left to right. With this transformation, the surrounding compact mesenchyme or developing ﬁbrous tissue no longer surrounds cartilage but bone and is thus known as periosteum. Because of this, the generalized statement that the perichondrium becomes the periosteum as soon as the ﬁrst perichondrial bone is formed has good morphological support and appears to be true for most bones. It is well to bear in mind that the transition from perichondrium to periosteum occurs at the time when it is still in a mesenchymal state and has not reached the stage of a specialized ﬁbrous tissue.
The cartilaginous otic capsule and its perichondrium
The petrous portion of the temporal bone with its contained otic capsule is one of the cartilage bones. It differs from other bones in that it is neither a long bone nor a ﬂat bone but rather a capsular or box-like bone enclosing the internal ear. The model or forerunner of this bone is the cartilaginous otic capsule. It makes its appearance in human fetuses of about 25 mm., crown rump length or at the age of 8 weeks. As the internal ear enlarges this capsule expands correspondingly. It remains cartilaginous until the internal ear has about attained its maximum size or at about the fourth or ﬁfth month, after which the cartilage stops growing except at one or two minor spots and these are irrelevant to the question under consideration. The cartilage of the capsule therefore has attained its maximum growth before ossiﬁcation begins.
The perichondrium during this period of growth and before ossiﬁcation has set in, has begun to develop into a specialized capsular type of ﬁbrous tissue. The innermost layer or two of cells next to the cartilage are somewhat rounded and resemble young cartilage cells. Outside of these the cells of the perichondrium become spread apart by the deposit of a collagenous ﬁbrous matrix. The outermost part of the perichondrium retains more of its mesenchymal character. This is especially shown in the center picture of ﬁgure 4.
The advent of ossification and the fate of the perichondrium
Ossiﬁcation centers appear in the cartilaginous otic capsule at various places. As many as fourteen such centers have been described. The ossiﬁcation center, like those in other cartilage models, are characterized into the calcifying cartilage P.B., The ﬁbrous layer transformed into perichondrial bone T.P.B., Trabeculae of perichondrial bone forming in the osteogenic layer by the enlargement of the lacunae and calciﬁcation of the matrix. The further transformation of the cartilage model has been previously described (Bast, ’30). Our attention here is directed to the formation of the perichondrial bone and the part played by tl1e perichondrium.
Fig. 1 Photomicrograph of a portion of the ossifying human otic capsule at the margin of the internal auditory meatus. (E. 38L —147 mm. (3.1%. — slide 23 - section 2 — Bast collection.)
Fig. 2 Photomicrograph of an area similar to that shown in the upper part of ﬁgure 1. -(E. 38L — 147 mm. 0.1%. - slide 22 - section 3. - Bast collection.)
Y.C., Young cartilage cell layer
F., Fibrous layer of perichondrium
0., Osteogenic layer of perichondrium
O.B., Osteogenic bud penetrating from the osteogenic layer through the ossifying ﬁbrous layer
Figures 1 and 2
The perichondrium overlying an ossiﬁcation center exhibits three or sometimes four distinct layers.
1. The young cartilage cell layer. This layer may be one to three cells in thickness. This layer, consisting of young cartilage cells with some collagenous ﬁbers between them, can be distinctly seen lying between the calciﬁed cartilage matrix of the ossiﬁcation center and the rest of the perichondrium. This layer quickly changes into mature cartilage a11d its matrix becomes calciﬁed and it thus becomes part of the cartilaginous ossiﬁcation center At the right in ﬁgure 2, this layer is seen as young cartilage whereas at the left, its matrix is being calciﬁed.
In the development of most other cartilage bone, it seems to be this layer which metamorphoses into an osteoblastie layer the cells of which multiply rapidly to form the osteogenie tissue that deposits perichondrial bone (ﬁg. 3).
2. The dense collaigcmlc layer. This layer of the perichondrium lies just outside the young cartilage cell layer. Its cells are spread apart and a dense matrix of collagenous fibers lies between them (figs. 1 and 2-F). This is the layer in wl1icl1 the ﬁrst perichondrial bone is deposited. It is not clear whether some of the cells in this layer actually become osteoblasts and deposit bone about themselves orwhether the calcareous deposit is dependent on the osteogenie layer just outside of this layer.
3. The mesenchymal layer. This gives rise to two sheets of tissue, (a) the osteogenie layer and (b) the periosteum.
(a) The osteogenic layer arises from the mesenchymal layer just outside the dense collagenic layer (ﬁgs. 1, 2, 4-0). Many capillaries invade this area and the cells are transformed into osteoblasts. The function of this layer is twofold; one, to give rise to osteogenie buds which will perforate the underlying layer and invade the modiﬁed cartilage or ossiﬁcation center, and two, to proliferate osteogenie tissue in situ and thus to give rise to the perichondrial bone. In areas where.peri— chondrial bone is being extensively formed, all of the mesenehymal layer may be transformed into this osteogenie layer (ﬁgs. 1 and 2, top).
(b) The periosteum as a rule develops from the outer part of the mesenchymal layer by the deposition of intercellular ﬁbers. It overlies the osteogenie layer and the developing perichondrial bone (fig. 4). In Fig. 3 A panoramic View of part of a rib of a woodchuck (Mossman collection) which shows the transformation of the primitive perichondrium into perichondrial bone and into periosteum. At the extreme left is shown the primitive perichondrium (P) consisting of compact mesenchymal cells overlying the rib cartilage (0). Reading the picture from left to right, one sees the transformation of the deeper layer of the perichondrium into osteogenic tissue and osteoblasts. Near the middle of the picture, at O.T., osteoid tissue is forming on the surface of the cartilage. Other trabeculae of osteoid tissue a.re seen forming in the osteogenic layer, at T.O.T. The dark staining osteobla.sts are lined up on the surface of these trabeculae. Toward the right the osteoid tissue is changed into bone. The periosteum (P.O.) is seen to be continuous with the original perichondriuin (P).
Fig. 4 This picture shows perichondrial ossiﬁcation of the human petrous bone. (E. 36-L——135 mm. C.R.——slide 10, section 2 -—- Bast collection.) The small center picture shows the perichondrium (P) of thecartilagenous otic capsule. It is comparable to the perichondrium of ﬁgure three except that next to the young cartilage cells the layer is ﬁbrous and not cellular. Both to the right and left, the picture shows various stages in the ossiﬁcation process and the zoning of the perichondrium and its transformation into perichondrial bone and periosteum. F.—Fibrous layer. O.———0steogenic layer. P.0.——-Periosteum. O.B.—Osteogenic bud penetrating from osteogenic layer through the ossifying ﬁbrous layer into the calcifying cartilage.
places where the mesenehyinal layer is all converted into osteogenic tissue, the periosteum later develops from the outer part of the osteogenic layer.
In the development of most cartilag'e bones, the cells of the perichondriurn which surround the carttilage model give rise to the osteo genic layer. The latter, in turn, forms the perichondrial bone. The outer, 1°eniai11i‘11g portion of the peroehondrium becomes the periosteum of the perichondrial bone.
In case of the petrous bone, the periehondrium of the otic capsule gives rise in its deeper part to a dense C0ll.a.g"€11lC layer which forms the basic tissue in which the first perichondrial bone is deposited. Most of the rest of the periehondrium gives rise to the ostyeogenic tissue which is primarily responsible for the forination of the perichondrial bone.
Only the outer part of the original perichondrium becomes the permanent periosteum.
Cite this page: Hill, M.A. (2020, June 6) Embryology Paper - Perichondrial ossification and the fate of the perichondrium with special reference to that of the otic capsule. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Perichondrial_ossification_and_the_fate_of_the_perichondrium_with_special_reference_to_that_of_the_otic_capsule
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